This top-secret spaceplane just returned to Earth, and nobody knows what it did up there

There’s plenty of manmade devices floating around in space, and between communications satellites, weather-monitoring cameras, and exploratory spacecraft like Cassini, the general public is kept in the loop as to why they’re all there. The same definitely cannot be said for X-37B, an unmanned spaceplane sent into Earth orbit by the US Air Force nearly two years ago. This weekend, the plane made its way back to solid ground, and the government is being particularly coy about what its mission was.

According to the US Air Force, the X-37B’s purpose is to act as an “experimental test vehicle,” and each of its missions — four of which have now been successfully completed — are described in exceptionally vague language.

“The landing of OTV-4 marks another success for the X-37B program and the nation,” Lt. Col. Ron Fehlen, X-37B program manager, said in a statement. “This mission once again set an on-orbit endurance record and marks the vehicle’s first landing in the state of Florida. We are incredibly pleased with the performance of the space vehicle and are excited about the data gathered to support the scientific and space communities. We are extremely proud of the dedication and hard work by the entire team.”

The official “fact sheet” for the X-37B program as a whole describes its overall goals as follows:

That’s a fine line for the PR department, and it’s not surprising that the US Air Force would be simply testing new space technologies in its efforts to maintain military superiority, but some skeptics aren’t buying it. Theories from all over the scientific community suggest the program’s ultimate aims are far more covert, ranging from spying on other nations to disrupting the spy satellites of competing countries. As with most military operations, the nitty gritty details are being held close to the vest, which means we may never actually know for sure.